The present invention relates to a corrosion inhibitor for use in coating formulations such as primers, intermediate coats or topcoats. The corrosion inhibitor comprises synergistic combinations of a calcium borate and an amino carboxylate, zinc bis[3-N,N-dipropylamine]propionate.
As is known in the art, corrosion inhibitors are incorporated into coating formulations to provide or enhance protection of underlying metal surfaces against oxidation and electrochemical corrosion. Known corrosion inhibitors include zinc chromate, strontium chromate, barium metaborate, calcium borate, zinc phosphate, calcium boro-silicate and zinc phospho oxide. These known inhibitors are all powdered pigments which are incorporated into coating formulations under high-shear agitation during the pigment grinding stage along with other common pigments such as titanium dioxide, calcium carbonate, clay, mica and barium sulfate.
Calcium borates are known corrosion inhibitors. Corrosion inhibiting coating compositions containing calcium metaborate are described in U.S. Pat. No. 3,380,836; the disclosure of which is incorporated herein by reference. However, when calcium borates are used by themselves in coating compositions they can cause the coating to be susceptible to blistering after being applied to a metal surface and being exposed to a corrosive environment. See Example.
The use as a liquid corrosion inhibitor of amino carboxylate which is zinc bis[3-N,N-dipropylamine]propionate, manufactured by CasChem, Inc. as COTROL 18-8 product has recently been reported in the trade literature. Unlike traditional corrosion inhibitors which are in pigment or powder form, the zinc bis[3-N,N-dipropylamine]propionate, being a liquid, is not incorporated by itself into coating formulations during the pigment grinding stage, but rather during the subsequent let-down stage along with other liquid components, i.e., resin emulsions, dispersants, etc. When the zinc bis[3-N,N-dipropylamine]propionate is used by itself to inhibit corrosion in a coating formulation, the coating formulation can be susceptible to softening of the final coating and loss of adhesion to the metal surface. Thus a coating formulation containing zinc bis[3-N,N-dipropylamine]propionate might be scraped off the metal surface to which it is applied, leaving the surface unprotected.